1. Field of the Invention
The present invention relates to an image forming apparatus in which it is possible to always form an image optimally.
2. Description of the Related Art
In conventional image forming apparatus such as color copying machines, light from an exposure lamp irradiates onto the original document. The reflected light from the original document pass through a RGB filter to obtain color signals each corresponding to color components. Then the color signals are treated to finally form a color image on a paper.
In image forming process such as this, there are various correction processes in the latter part of the image density conversion, when a standard paper, which is used as a paper having a white color, is placed on the original document table, it is assumed that the image signal is optimized at 1.
Consequently, at the point in time when the standard paper is placed on the original document table, when there are large differences in brightness and color from a shading correction standard plate mounted on the original document table, the assumption of the image signal being at 1 is void.
Furthermore, as this type of image treatment continues as it is, the above-mentioned assumption of the brightness of the image is inaccurate and as the various correction treatments are undertaken in practice, in extreme case there are fluctuations in the color matching such that the quality of the image is noticeably deteriorated.
To overcome the above-mentioned problem, in a color image reading apparatus shown in U.S. Pat. No. 4,891,692 (Jan. 2, 1990), a CPU refers the reading image signal value, then the CPU calculates optimal density conversion parameters based upon this reference value to correct the image density conversion parameters. After that, the process of image density conversion is performed.
In the above-mentioned case, if the apparatus have fluctuations of characteristics of the exposure lamp, fluctuations of precision of the construction, and etc., the brightness of the image is not converted uniformly within every pixel of each of the R, G, B colors signals in many cases.
In this way when the ratio of the sensitivity of each R, G, B color signal is thus distorted, in prior art correction processes, the problem of the image quality fluctuations between each apparatus can not be solved.